The present invention relates to a light scanning apparatus which scans a light beam having coherence on a scan surface, and furthermore to a scanning display apparatus which forms an image on the scan surface by light beam scanning.
A variety of scanning display apparatuses have been suggested in which images are projected on a screen by scanning a light beam having coherence such as laser beams.
Japanese Patent Laid-Open No. 2003-021800 discloses an example of a laser scanning display which displays a color image on a screen by scanning laser beams of three colors that are red, green, and blue in a two-dimensional direction by using a deflector. The laser light source is suitable as a light source of the scanning display apparatus due to the height and directivity of the output. However, since the coherence is high in the laser light source, a speckle noise (granular interference pattern) that is specific to the laser appears in images, and the sharpness of the image deteriorates.
As a technique for reducing the speckle noise, Japanese Patent Laid-Open No. 2005-084117 discloses a method in which a movable diffuser is arranged between a light scanning device and a projection lens, and time phase modulation is given to scanning light which passes the diffuser. In this method, the phase is modulated in each frame, and a different speckle pattern corresponding to the phase modulation is generated and superposed in the temporal resolution of the eye to reduce speckle contrast.
Japanese translation of PCT International Application No. 2001-509911 discloses another method for reducing the speckle noise. Specifically, it discloses a method in which a laser beam from a light source is divided into a plurality of partial beams by using a phase hologram, and the plurality of partial beams are moved in one pixel to generate a different speckle pattern so that the speckle pattern is averaged in time and space.
However, according to the method of reducing speckle disclosed in Japanese Patent Laid-Open No. 2005-084117, when the frame rate is set at 60 (Hz) and the temporal resolution of the eye is set at 1/30 (sec), there are two kinds of speckle patterns which occur in the temporal resolution of the eye. In this case, the speckle contrast only decreases to about 71%, and the speckle reduction effect is low.
In the scanning display apparatus, two dimensional scanning of an imaged beam spot is performed on a screen. However, only one beam spot is projected on the screen at a certain moment, and the two-dimensional image which is drawn by a beam spot for each moment is recognized due to the afterimage effect of the eyes.
Thus, since the time in which a beam spot exists in a certain pixel position is an instant, even if the diffuser is moved fast, at the point in time when the diffuser has arrived at the position where the phase changes, the beam spot will transfer to another position. In this case, even if the diffuser is moved fast, there is not a substantial speckle reduction effect, and only a speckle reduction effect equal to the phase modulation of each frame is obtained.
Generally, in a scanning display apparatus, the size of one pixel is the same size as the diameter of the beam spot imaged on the screen or a size slightly smaller thereof.
However, according to the method of reducing speckle disclosed in the Japanese translation of PCT International Application No. 2001-509911, the scope which is irradiated by a plurality of mutually parallel partial beams is set as one pixel and the plurality of partial beams is collected with a phase hologram to form a plurality of beam spots in one pixel. When this is replaced with a beam spot diameter equal to one pixel size, the region of one pixel expands, and the resolution deteriorates.